I. Field of the Invention
The present invention generally relates to methods for converting alkanes and alkenes to, respectively, alcohols and diols. More specifically, the present invention is directed to conversion methods using liquid and gas phase reactions with liquid reactants which are regenerated and recycled in the process.
II. Description of the Background
Methane has previously been converted to methanol by the halogenation of methane followed by hydrolysis of the methyl halide to form the alcohol. For example, gaseous chlorine has been used to chlorinate methane to form chlorinated methanes, principally methyl chloride, together with other chlorides, i.e., dichloromethane, trichloromethane and carbon tetrachloride. Alternatively, methane has been subjected to oxychlorination with oxygen and hydrochloric acid to form those compounds. The chlorinated methanes produced are hydrolyzed in the vapor phase to produce methanol, formaldehyde, formic acid and by-products, including carbon dioxide and hydrochloric acid, depending on the chlorination selectivity. Hydrochloric acid is produced or used in the halogenation of methane by either method and must be recovered, dehydrated by azeotropic distillation and recycled. Corrosion and problems involved with the handling of chlorine and hydrochloric acid are substantial. Many have tried to solve these problems.
U.S. Pat. No. 3,172,915 to Borkowski, et al. proposed a process for converting methane to methanol. Borkowski disclosed the chlorination of methane using ferric chloride at high temperatures to produce chloromethanes and hydrogen chlorides. This chlorination required temperatures in the range of 220-800.degree. C., more preferably 250-450.degree. C., and long residence times, e.g., more than one hour. Further, this process was hindered by the production of a mixture of chlorination products, e.g., chloromethane, dichloromethane, trichloromethane and carbon tetrachloride, which must be separated before hydrolysis to produce methanol. Other disadvantages result from the energy required to dry the ferric chloride and from the corrosion and handling problems inherent with hydrochloric acid.
U.S. Pat. No. 5,243,098 to Miller disclosed another method for converting methane to methanol. Miller disclosed the reaction of methane with cupric chloride to produce chloromethane and hydrochloric acid. These intermediates were then reacted with steam and a catalyst containing magnesium oxide to produce methanol and magnesium chloride. Magnesium oxide was regenerated by contacting the magnesium chloride by-product with air and oxygen. Cupric chloride was regenerated by contacting the cuprous chloride by-product with air and hydrochloric acid. While these reactions proceed at favorable rates, attrition of the solid reactants, i.e., cupric and magnesium oxide, was significant. Special filters and processes were required to recover and regenerate these reactants in the required particle size. Miller also suggested the use of cupric bromide and magnesium zeolite as alternative reactants. Because of the attrition of the reactants, difficulties associated with the handling of solids and the special filters and processes required to regenerate the reactants, this process has proved unsatisfactory. U.S. Pat. No. 5,334,777, also to Miller, disclosed a substantially identical process for converting ethene to ethylene glycol.
While the foregoing processes have disclosed methods for converting alkanes and alkenes to, respectively, alkanols and diols, those skilled in the art have continued to search for commercially viable processes, particularly commercially viable processes for converting methane to methanol. Such processes are particularly desired by the oil and gas industry in order to reduce the high costs of handling, transporting and storing natural gas. These costs can be reduced significantly if the gas, principally methane and ethane, is converted to methanol and ethanol. Economical and efficient processes for such conversion would find wide use in remote gas fields, e.g., in the North Slope of Alaska, in the North Sea and at other offshore locations.
Thus, there has been a long felt but unfulfilled need for more economical and more efficient methods for converting methane and other lower hydrocarbons to, respectively, methanol and corresponding alkanols and diols. The present invention solves those needs by providing methods with faster reaction kinetics, lower operating temperatures, fluid (liquid and gas) reactants which are easily pumped through the process equipment, efficient regeneration of reactants and little or no attrition of reactant by using liquids which can be readily pumped and recovered.